(1) Field of Invention
This invention relates to mixers using MOSFETs, in particular the down converter of a superheterodyne radio receiver
(2) Description of the Related Art
In a conventional radio receiver, the incoming radio frequency is mixed with a local oscillator (LO) signal to produce a beat frequency, which is the intermediate frequency (IF). The IF is then amplified and filtered to attenuate other unwanted signals,
A popular mixer circuit is the Gilbert multiplier. Since MOSFETs are widely used in circuit designs today, an MOSFET version of the Gilbert multiplier is shown in FIG. 1. Basically, a differential amplifier with a differential pair N2 and N3 is fed from a current source N1. The differential gain of the differential amplifier is proportional to the transconductance gm of N2 and N3. This transconductance varies as the square root of the dc drain current of N2 and N3, which is controlled by the drain current of N1. The dc drain current ID of N1 is controlled by the dc gate-to-source voltage VGS1 of N1 and has a square law relationship with the gate-to-source voltage i.e. ID1xe2x88x9d(VGS1xe2x88x92Vt)2, where Vt is the threshold voltage of N1. When a local oscillator signal VLO of frequency fLO is applied differentially to N2 an N3 (i.e. VLO+ and VLOxe2x88x92 respectively), and a radio frequency signal Vrf of frequency frf is applied to the gate of N1, the output current of the differential amplifier is equal to VLO*gm, and the gm is proportional to Vrf*(VGS1xe2x88x92Vt). When the Vrf is multiplied by VLO, a beat frequency fif=frfxc2x1fLO intermediate frequency signal Vif is produced.
While the Gilbert multiplier is widely used in the past, it has a number of drawbacks for low voltage and low power applications. In modern CMOS technology, the tendency is to use a low supply voltage VDD: for instance 25 V for 0.25 xcexcm technology and 1.8 V for 0.18 xcexcm technology. In the Gilbert mixer, the current source is operating in the current saturation region of the VDS vs ID V-I characteristic N1xe2x80x2 in FIG. 2 to obtain a higher transconductance and has a square law relation with Vrf. Therefore the drain voltage VDS1 for the current source N1 is larger than the knee voltage VD1xe2x80x2 of the N1xe2x80x2 V-I characteristic curve. That knee voltage VD1xe2x80x2 is equal to VGS1xe2x88x92Vt.
Similarly, the differential pair N2 and N3 also must have its drain voltage higher than the knee voltage, i.e. VD2 greater than 2(VGS1xe2x88x92Vt) as shown by the dotted V-I characteristic of N2 in FIG. 2. If a resistor is used as a load, another voltage drop VL will be added to VDS1 to be supplied by the power supply VDD. These three stacks of voltages, VDS1, VDS2 and VL, dictate that the supply voltage cannot be made very low. For a typical threshold voltage of 0.6 V, there is hardly any xe2x80x9chead roomxe2x80x9d for signal voltage swing.
Lee et al disclosed in U.S. Pat. No. 6,194,947 a mixer structure which is basically a Gilbert mixer having a differential pair fed from a current source with its shortcomings.
Sakusabe disclosed in U.S. Pat. No. 5,789,963, FIGS. 1 and 9xe2x80x9d, a mixer operating with a drain to source voltage VDS in the current saturation region of a MOSFET without claiming the exact VDS. The RF signal is injected to the drain of the mixer by AC coupling (i.e. through a coupling capacitor). The AC coupling requires many additional components such the coupling capacitor Ca and other components such as Z1-Z8 and capacitors C1-C5 as shown in Sakusabe""s FIG. 1
Another drawback of Sakusabe""s mixer is that the gate of the mixer FET2 must be adjusted to set the quiescent drain voltage to the current saturation region for different operating currents. It is desirable to set the set the quiescent operating point (i.e. VDS) automatically for different operating currents.
An object of this invention is to design a MOSFET mixer which requires a lower supply voltage than the Gilbert mixer or similar structure. Another object of this invention is to reduce the power consumption of the MOSFET mixer. Still another object of this invention is to provide a high conversion gain of the mixer. A further object of this invention is to set the operating point of the mixer at its optimum conversion gain automatically.
These objects are achieved by mixing the RF signal and the local oscillator signal at the knee of the output VDSxe2x88x92ID characteristic of a MOSFET by dc coupling. At the knee, the characteristic has the sharpest curvature. The nonlinearity produces a maximum beat frequency signal. For implementation, a mixer MOSFET is biased at the knee of the VDSxe2x88x92ID characteristic. The LO (or RF) signal voltage VLO (or Vrf) is applied at the gate of the mixer MOSFET, and the RF (or LO) signal voltage Vrf(or VLO) is injected at the drain of the mixer MOSFET. Then a beat frequency drain current is produced. Specifically, the gate of a single-ended mixer MOSFET is fed with a local oscillator signal and the drain of the mixer is dc coupled to a single-ended source follower with the gate fed from a radio frequency signal or vise versa.